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# Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

## Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

(OP)
For example, I must install a fan with a dust filter (point 3) in this branched pipe system.

The pressure drop from point 1 to 3 is bigger than the point 2 to 3. Both are open to the atmosphere.

For calculate the mmWG I have to reach I will sum the pressure drop from 1 to 3, the pressure drop of the filter and the discharge pipe.
Why I do not shall sum the pressure drop from 2 to the junction???

Honestly, I do not understand why, I think that the fan must reach the pressure drop of all not only the worst case…

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

I think you are trying to design a supply from point 1 to filters at both 2 and 3. Both filters operate at the same time. Flow to filter 2 = Q2. Flow to filter 3 = Q3. The flow in 1-B will be Q1 = Q2+Q3.
There will be a pressure drop calculation for each route, 1-2 and 1-3.

Each route 1-2 and 1-3 include a pressure drop from 1 to the branch point, then from the branch point (B) to B-2 OR B-3.

For the 1-2 route, sum pressure drops from 1-B plus B-2.
Calculate the pressure drop in 1-B using Q1.
Calculate the pressure drop in B-2 using Q2. So yes, you must calculate pressure drop in B-2.
That is the pressure drop from 1-2.

For the 1-3 route, sum pressure drops fom 1-B plus B-3.
You already know the pressure drop in 1-B using Q1.
Calculate the pressure drop in B-3 using Q3. So yes, you must also calculate pressure drop in B-3.
That is the pressure drop from 1-3.

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

(OP)
Okay 1503-44,

Now i know the pressure drop in each branch, correct,

to select the fan or pump height, I have to choose the worst case or 1-B-3 plus 2-B?

thats my query

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

It's not clear what direction this flow is in but assume it's towards the filter/pt3.

The point is that the flow will be different in the two legs so the pressure drop will actually be the same in the two legs unless you turn one off.

The pressure at point B will be the same. If the ends are open to atmosphere the pressure drop will be the same in both legs.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

SF6,

Now I'm not totally sure what you are trying to do. I thought you had fans at points 1 and 2, but now LittleInch has a point with his question that we need to have clear.

Do you want to pull air from both points 1 and 2 with the fan at point 3?

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

(OP)

#### Quote (1503-44)

Do you want to pull air from both points 1 and 2 with the fan at point 3?

yes

#### Quote (LittleInch)

The pressure at point B will be the same. If the ends are open to atmosphere the pressure drop will be the same in both legs.

that is what i do not undestand, why?? why the worst case?

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

OK.

Pressure drop will be the same from 1 to 3 as it is from 2 to 3, because you have atmospheric pressure at both points 1 and 2. That's 14.7psia, more or less. You also will have a drawdown pressure at the fan at point 3, say it is 12.0psia. You can't change atmospheric pressure at 1 or 2, so the pressure drop between 1-3 is 14.7-12 = 2.7psi
Same situation between points 2-3. 14.7-12 = 2.7psi.

So then what happens is flow from 1 and the flow from 2 will be different. The flows will add together at the B and continue to the fan at 3.

The problem becomes how much flow can you get from 1 and from 2.
Assume some flow rate at the fan, Qf. Find its drawdown pressure with that flow.
With Qf in the duct B-3, find the pressure drop in B-3.
Now divide Qf by 2 and put half the flow in 1-B and half in 2-B.
Find the pressure drop in 1-B with Qf/2.
Do the same for 2-B.

Add the pressure drops for 1B and B3 together. The result must not be greater than 2.7psi.
The same is true for 2B+B3, but we will get to that later.
If it is greater, then reduce Qf a small amount. If it is less than 2.7psi, then increase Qf a small amount.

Add or subtract 1/2 that same small amount to 1B and to 2B.
When you finally get 2.7psi pressure drop in 1B3, do the same thing with 2-B-3.
Eventually you will have a 2.7psi pressure drop in each route, both 1B3 and 2B3.

Now if you put dampers at the intakes, you can reduce atmospheric pressure there a little as necessary.

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

It's not the "worst case", its the only case as its the same for both lines as explained by Mr 44.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: Query: Why do I have to take into account only the worst case and not the sum of all pressure drop?

(OP)
I felt very silly for not understanding it, but that's it! thank you all very much,

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